Molten slag at high temperature is usually produced in the smelting of ores and in the fining or refining of raw metal. The tapping of the slag removes heat from the system at a high rate and the liquid slag is cooled and assumes a solid state in a relatively short time and can then be handled, with some difficulty.
In general, the slag has limited economic value. Only a small part of the slag is used as a building material and a major part may have to be dumped as a waste material, although it contains substantial thermal energy, and is lost for the recovery of the heat that has been removed from the system.
As the recognition of economical problems and of the need to save energy has increased, numerous efforts have been made to direct more attention to the recovery of energy in molten waste slag.
Japanese Patent 61-08 357 (C.A. Vol. 105, Ref. 9845 y) discloses, for the granulating of slag, an apparatus that consists of a compact drum. Water-cooled wings are attached to a central shaft and are reversibly rotated to divide the slag. The bottom half of the drum is cooled by flowing water and the water which has been heated is delivered to a plant for a recovery of energy. The drum has a lateral inlet and an outlet for discharging the granulated slag.
In the process disclosed in Japanese Patent 58-199,034 (C.A. Vol. 100, Ref. 142, 883 x), blast furnace slag is granulated in a horizontal drum, which is provided with an internally cooled screw-like shaft and with a water-cooled wall. The slag is charged onto the rotating shaft and is dispersed and solidified and as granulated slag is removed through an opening formed in the bottom close to the end opposite the slag-charging end.
The known processes do not always satisfy the requirements or commercial practice and also, have the disadvantage that they can be performed only with difficulty in practice.